【答案解析】Now, millions of Americans have become obsessed with staying slim and "in shape". The pursuit of a youthful physical appearance is not, however, the sole reason for America's fascination with diet and exercise. Recent research has shown the critical importance of diet and exercise for personal health. As in most technologically developed nations, the life-style of North Americans has changed dramatically during the course of the last century. Modem machines do all the physical labor that people were once forced to do by band. Cars and buses transport us quickly from point to point. As a result of inactivity and disuse, people's bodies can easily become weak and vulnerable to disease. In an effort to avoid such a fate, millions of Americans are spending more of their time exercising. The effect of this new appreciation of the importance of exercise is evident: parks are filled with joggers and bicyclists, physical education programs are enjoying a newly found prestige, and many companies are providing special exercise facilities for their employees to use during the work day.